Real-time monitoring of structural response.
Offshore drilling operations are becoming more complex and moving to more challenging locations. Operators are responding by increasing their focus on asset performance as physical challenges such as water depth, harsh environments and high pressures and temperatures present more engineering challenges.
Alex Rimmer, director, 2H Offshore, and Edward Elletson, marketing coordinator, Pulse Structural Monitoring, talked to S2S about how they helped one operator to a clearer understanding of asset fatigue life by installing an advanced monitoring system on a North Sea jackup rig.
When the operator began development drilling and completion activities in the North Sea, the 90-m water depth meant that the jackup rig it was using was close to the limits of its operational capabilities. This reduced the safety margin in the structural integrity of the jackup rig, the riser and the subsea wellhead.
“An analysis of the high-pressure riser and wellhead conducted before drilling showed that the large motions the jackup rig experienced during service meant that the high-pressure riser was subjected to high loads, stresses and levels of fatigue damage,” Rimmer says. “Furthermore, the rig had high loading at the interfaces with the riser equipment. These findings revealed several potential structural integrity threats to the riser.
“In view of this and the operating guidance for the riser based on metocean data, the operator decided to implement a riser integrity management system using structural monitoring to measure the in situ riser response. This would provide data for verifying and calibrating the analysis model used as a basis for design decisions and ensure that the integrity of the riser was retained throughout drilling and completion.”
The system instrumentation, which was installed above the water line, had four main components:
“We hard-wired the instrumentation back to a central control unit,” says Elletson. "Here, the raw data is calibrated into acceleration, pressure, angular rate and strain, and then processed using the proprietary software drillASSURE to the required outputs of displacement, rotation, tension, bending moment and fatigue damage in real time. Traditionally, these monitoring systems are standalone: the data is retrieved after operations conclude. However, using the latest technology, Pulse can enable operators to measure and analyse structural responses in real time and assess riser integrity on an ongoing basis.”
The system includes a communication link that enables data transfer to onshore servers for further analysis and remote system monitoring and maintenance. Operators can also define, monitor and display key performance indicators and set alarm states that will be triggered if safe levels are exceeded.
“Using the data collected, the operator can monitor the structural response of the high-pressure riser system in real time and compare it with the predicted response under the measured environmental conditions to verify the analysis model,” Elletson explains.
“In this case, the comparison showed underprediction of fatigue damage in the model for several reasons,” Rimmer continues. “Some of the conservatisms usually present in analysis models of riser systems had been reduced on the basis of sound engineering principles to ensure that an acceptable design was achieved.
“The measured fatigue damage incorporated all sources of fatigue loading, whereas the prediction relied only on wave-induced fatigue damage accrual. The modelled jackup motions were generally underpredicted at wave heights of about 2–3 m. These are significant because they are the most critical for fatigue damage accrual, as they occur most frequently in the North Sea.
“The analysis model used to evaluate the riser design before operations did not represent all the stages of operation. Specific operational activities can have an impact on the overall fatigue damage, but are generally not factored into the design process,” he concludes.
There are several options for alleviating these factors and calibrating the model better against measured fatigue damage. However, full calibration for a single project is impractical because of the number of potential variables. Instead, a monitoring campaign indicates the “calibration factor” (or experience factor) between the theoretical model and the in situ riser.
“In this instance, the measured data showed that a calibration factor of approximately 30 is required for intervention operations, i.e., the predicted fatigue damage from the software must be multiplied by 30 for a better prediction of the fatigue damage,” explains Rimmer. “Put another way, the initial predictions of design fatigue life for the system need to be reduced by a factor of 30 during intervention operations for this rig. The calibration factor is location specific and it changes, depending on the operations through the riser.
“Despite the higher fatigue damage accumulation rate for intervention operations, our monitoring system demonstrated to the operator that the equipment was operating within its design limits during this work, as the overall fatigue damage accumulated in the riser was less than 1% because of the relatively calm conditions that occurred during the operations.
“Hence, there was 99% of the design fatigue life still available for further intervention activities or other operations, so there was no need to inspect the riser or changeout the riser joints to continue operations on the well. However, we now know that intervention operations have increased the rate of fatigue damage accumulation. Although not necessary in this instance, further review and analysis of this operation type could enable suitable mitigation measures to be put in place to reduce fatigue accumulation to the riser,” Rimmer concludes.
Using structural monitoring offers improved management of riser integrity. In this case, the monitoring system has demonstrated that the analytical models used during engineering required calibration and that, despite using conservative design methods the modelled response may not be conservative compared with the real response.